1. Field of the Invention
The present invention relates to a sewing machine having an automatic needle thread control device and a thread breakage detecting device.
2. Description of the Related Art
A conventional sewing machine having an automatic needle thread control device is, for example, disclosed in Japanese Patent Publication No. 47869/1985. In this sewing machine, the feed amount of the needle thread necessary and sufficient for forming a stitch in a work fabric is calculated based on a needle swing amount and the feed pitch, and the thickness of the work fabric, and the thus calculated amount of the needle thread is fed out by means of rollers driven by a pulse motor.
A conventional sewing machine having a thread breakage detecting device is, for example, disclosed in Japanese Patent Publication No. 9191/1984. This type of sewing machine includes a thread detecting circuit for detecting movement of a thread take-up spring which is put into engagement with a needle thread by means of the thread take-up motion of a thread take-up lever, thereby generating a thread detection signal. In order to prevent a malfunction, it is determined that a thread breakage has occurred and a motor is caused to stop when the thread detection signal has not been generated even once while the main shaft of the sewing machine has made a predetermined number of rotations.
However, if such a thread breakage detecting device is attached to a sewing machine having the related art automatic needle thread control device, there arises the following problem. At the time when sewing has just been started, over casting is made, or a thin fabric is sewn with a slippery, thin thread, the needle thread between the automatic needle thread control device and the needle may become loose. Even when the needle thread is in such a loose state, the automatic needle thread control device feeds out the needle thread continuously and, hence, the needle thread does not recover from the loose state. As a result, even if the thread take-up lever makes the thread take-up motion, the needle thread will not engage the thread take-up spring while the main shaft makes a predetermined number of rotations. Thus, the thread detecting circuit detects no movement of the thread take-up spring, and therefore, such a problem arises that the thread breakage detecting device mistakes the loose thread for a thread breakage and thus stops the main motor.